: This may be an incredilbly stupid question but....
: What would happen if the Tain was distroyed?
: Is there a way that all of these beings
: "trapped" in the Tain could still
: "be" if it had been distroyed?
: Was it distroyed?
Well, if the Tain is merely a portal into an alternate universe, then nothing much would happen. As it happens, in TFL, the Tain is only shattered with each shard still able to work as designed.
If however, the universe is contained within it, I believe that everything inside it would also be destroyed. This would also explain the regions of void you see in The Summoner as areas running along the shatter lines. The part of the Tain which is needed to make those regions whole is in another shard so instead, there is nothing. This nothingness is not a portal back into Mythworld so anything caught within it will not be returned there.
Therefore, if it is entirely destroyed, the whole thing will be nothing (if you see what I mean) so everything inside it will cease to exist in either universe.
Now you might say, "Why didn't Connacht just destroy the Tain after trapping the Myrkridia within it?" but I assume that he would want to hold on to it due to its effectiveness against hordes of unstoppable nightmares.
